Conventionally, various image forming apparatuses having the image reading function have been proposed. For example, in an image forming apparatus disclosed in Japanese Patent Application Publication No. 9-200399, an original reading unit has a casing which accommodates an imaging device such as an image sensor therein, and an original tray and an original discharge tray are fixed to a side wall of the casing so that ends thereof contact the side wall. When an original is sent from the original tray, an image on the original is read in the casing. Then, the original is discharged onto the original discharge tray.